1. Field of the Invention
The present invention relates generally to a method and apparatus for prognosticating unexpected cardiovascular disorders, such as cardiac arrests of patients having or suspected of having one or more cardiac diseases. The method of the invention involves calculating an index indicating the risk of a sudden cardiac death. Below, this index is termed the Sudden Cardiac Death Risk Index (SCDRI).
2. Description of the Related Art
State-of-art physiologic monitors provide a large variety of different parameters alerting the clinical and nursing staffs to lethal events of patients. However, the algorithms for calculating each parameter monitored require time from the onset of the lethal event, such as ventricular fibrillation (VF), Torsades de Pointes (TdP), or ventricular tachycardia (VT), before the alarm is given. Thus, there is typically a delay before the clinical staff is alerted to arrange the necessary therapy, e.g. defibrillation during lethal events with the patients. In the worst case, when only the electrocardiogram (ECG) signal is monitored, the delay may be as high as 60 seconds before the relevant alarm of a lethal event is given. At present, the nursing staff has the responsibility of making the prognosis based on their experience in interpreting the massive amount of parameters in acute care (acute care refers to a level of health care in which a patient is treated for a brief but severe episode of illness, for conditions that are the result of disease or trauma, or during recovery from surgery). Given the facts that every minute from the onset of a cardiac arrest reduces the survival chances of the patient by 7 to 10 percent and that irreversible damage starts to occur within 4 to 6 minutes, the experience of the nursing staff is vital.
Clinical studies have shown the non-linear behavior of heart function and control mechanisms. Several parameters have been identified that indicate the baro-reflex sensitivity (BRS) and the dynamics of the autonomic and vagal control mechanisms of the heart. Some of these parameters, such as heart rate variability (HRV), have been studied and proved to have value in predicting a cardiac arrest. For example, depressed function of the cardiovascular control system is seen as decreased chaos in the HRV. This means that there are less chaotic but more periodic frequency components in the HRV. In other words, a healthy heart rhythm is chaotic showing a fractal form, which is broken by an abnormality or disease.
One method based on the detection of chaos is disclosed in U.S. Pat. No. 5,769,793. In this method, a quantity called “approximate entropy” is determined based on the medical data measured, the “approximate entropy” indicating the degree of chaos in the behavior of the human body, for example. The method may be used, for example, in the analysis of electrocardiograph data, such as beat-to-beat heart rate data derived from an ECG signal.
One drawback with the above-mentioned method is that it studies the entire behavior of the cardiovascular control mechanism, but not the local disorders of the myocardium. If the patient suffers from poor perfusion, adequate oxygenation and correct energy balance cannot be maintained for the myocardium, which may lead to cardiac ischemia and to an acute infarct. In order to arrange an adequate therapy and to avoid lethal injuries in the heart, it is thus crucial to have knowledge of the current local processes in the myocardium. A reliable indirect method for measuring the myocardial oxygenation is the examination the ST segment level, for example, of the ECG of the patient.
In this connection, reference is made to FIG. 1 that shows one cycle of an ECG signal. As is commonly known, and also shown in the figure, the waves of the ECG signal (i.e. the depolarisation and repolarisation events in the heart) are named alphabetically from P to U. Modern ECG devices use digital signal processing to analyze the shape and the consistency of, and the durations between these waveforms. In addition to the ST segment level, the examination of the T wave morphology and the QT duration are also highly valuable in estimating the energy balance and the ion pump function of the myocardium and its cells, i.e. local disorders of the myocardium. Proarrhythmia drugs may prolong the QT duration, which has been found to increase the risk of TdP and VF, and sudden death. Furthermore, lack of oxygen and an electrolyte imbalance may cause ventricular arrhythmias and bundle branch blocks. These life-threatening phenomena can be seen on the ECG as changes in the ST segment level and in the T wave amplitude. Detection of rhythm and conduction abnormalities, such as bundle branch blocks, require measurement of P, QRS, and T wave intervals and amplitudes. This can be implemented by commercially available interpretation algorithms, such as the Glascow Royal Infirmary program. Continuous monitoring and comparison of the ECG reveals the propagation of abnormal events.
Many scientific studies have also been published, which aim to find a diagnostic method for identifying the patients with the risk of a sudden cardiac death. One of such studies is disclosed in US Patent Application 2002/0138012 that discloses a method for identifying the patients with increased risk of having an episode of Sudden Cardiac Death syndrome (SCD) and thus in the need of receiving an implantable cardioverter-defibrillator (ICD) to reduce the risk. In these methods, the data is collected by specific diagnostic devices, such as ECG cards or ambulatory Holter devices featuring 24 hours monitoring, ECG storage and off-line analysis. The collected and stored data is analyzed off-line by separate computers.
These screening systems have not been designed for acute care, where continuous and real-time monitoring is a basic and fundamental requirement.
It is the objective of the invention to provide a mechanism for alerting in advance of the onset of a cardiac event, such as cardiac arrest, of a patient in acute care.